1. Field of the Invention
The invention relates to gas sensors.
2. Discussion of the Background
Gas sensors are useful for safety monitoring, process monitoring and control, evaluation of gaseous product quality, environmental control and automotive applications ranging from analysis of (i) engine air-to-fuel ratios, (ii) exhaust, (ii) passenger's compartment, and (iii) to breath tests. There is a significant need for inexpensive, selective and reliable gas sensors for performing these functions.
It is known to detect a species in a composite gas using a thin film of a suitable semiconductor material whose electrical resistivity changes in response to a presence of the species. For example, a semiconductor tin oxide thin film may be used to detect nitrogen oxides NO.sub.x. See U.S. Pat. No. 4,169,369. The electrical resistance of the film exposed to the gas is measured and provides a basis for determining the concentration of the species. The responsiveness of the film is temperature dependent. In general, it is necessary to heat the film to a predetermined elevated temperature to optimize the sensitivity for the particular species.
The delicate thin film is carried on a substrate having a substantially greater thickness to permit convenient handling. It has been proposed to mount a plurality of semiconductor thin films on a common substrate. For example, a second film may be employed to detect interference due to a species other than the primary species of interest. The films, which may have like or unlike compositions, may have distinct sensing temperatures.
The substrate may also include an integrated circuit for interpreting the electrical resistance measurements, the operation of which may be adversely affected by heat. To better control temperature at other sites of the sensor, it is desired to limit the heating of the film to the immediate location of the film. However, heating the film necessarily results in heat loss to the underlying substrate, which tends to conduct heat to other locations of the device.
U.S. Pat. No. 4,706,493 discloses a gas sensing device having a tin oxide thin film deposited onto a silicon dioxide layer overlying a heating element. This tin oxide thin film comprises a gas contact surface carrying a thin, discontinuous palladium-gold catalytic film to enhance sensitivity. In this device the tin oxide film is the situs of the gas detection region of the device.
To date, however, gas sensing devices have not been very selective and frequently show failure mechanisms that preclude long term use. In particular, devices based on film conductivity, including SnO.sub.x devices, have the following problems: low sensitivity (ability to detect gas of interest) and failure to quantify gas present, low selectivities (ability to detect the gas of interest in the presence of other gases), long-term drift, hysteresis effects, limited range of gases that can be detected, limited range of operating temperature and slow responses.
Accordingly, there is a strongly felt need for a gas detector which does not suffer these disadvantages.